Means For Applying Tension To A Top Tension Riser

ABSTRACT

A means for applying a controlled tension to a top tension riser on a floating offshore structure. The invention combines a mechanical tensioner device and buoyancy can to apply controlled tension to an individual riser or group of risers supported by a floating offshore structure. The buoyancy can applies static tension force on the riser(s). The mechanical tensioner applies additional tension force to assist the can in limiting the stroke of the riser as the supporting structure is displaced from its nominal position

RELATED APPLICATIONS

This application claims and references the benefit of ProvisionalApplication Ser. No. 60/798,091 filed on May 4, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is generally related to risers used on offshore structuresand more particularly to means for controlling tension in the risers.

2. General Background

In the offshore production of oil and gas (fluids), offshore hostfacilities can be one of many different types, such as TLP's (tensionleg platforms), Mini-TLP's, Spars, Semi-submersibles, etc. These hostfacilities most often bring production fluid onboard and produce thefluid for export through a pipeline or floating transport vessel. Thefluid comes from subsea wells either directly to the host facility orvia manifolds that commingle the production fluid from several differentwells, prior to the fluids being brought onboard a host facility.

The means typically used to bring the production fluids to the hostfacility include steel catenary risers (SCR's), flexible flowlines, toptensioned risers (TTR's), and free standing risers.

Steel catenary risers (SCR's) are essentially a pipeline that hangs offthe side of the host facility by way of a flex joint or a pull tube.Installation involves a lay barge, which drives up installation costs,as field welding is required to manufacture the SCR. The weight of theSCR imparts a high hang off load onto the host facility. There are alsofatigue issues associated with host facility motions being transmittedinto the SCR.

Flexible flowlines are a multi-layer flexible hose that hang off thehost facility via a collar. The hoses hang in a catenary shape similarto an SCR but with a more dramatic sag. The flexible flowline offers thebenefit of faster installation times relative to the SCR, as there areno in-field welds that must be made. Typically, the flexible flowline isreeled out during installation. The flexible flowline can also reducethe payload imparted onto the host facility, as the departure angles forflexible flowlines are smaller than SCR's, which yield a shorter freehanging catenary length, reducing the weight, which is another benefit.However, flexible flowlines are expensive to manufacture relative topipe. Not only is the manufacturing cost high, but the flexibleflowlines have temperature and pressure limitations relative to steelpipe.

Another alternative is a top tensioned riser (TTR). These risers aremade of steel pipe with various specialty stress joints and connectorslocated at the sea floor and the keel of the host facility. Thesespecialty joints help reduce localized high bending loads generated inthese areas. The weight of the TTR is either supported by the hostfacility via tensioners or air cans that provide buoyancy independent ofthe host facility. Installation of the top tensioned riser isaccomplished via a rig located on top of the host facility. The subseawellhead that a TTR is tying back to the host facility must be locatedwithin a relatively small horizontal offset distance from the well slotwhere the TTR enters the host facility such that the TTR remain almostvertical during all operations. This is one of the main restrictions ofa TTR. The main advantage of the TTR is that it allows direct accessinto the subsea well from the host facility. This access allows theoperators to stimulate the well and increase the amount of productionfluid that can be recovered from a single well. If such intervention isrequired on wells with the other riser types, a separate vessel known asa Mobile Offshore Drilling Unit (MODU) has to be mobilized at a highcost to carry out the work. Sometimes this cost does not justify theintervention and the well is abandoned with otherwise recoverableproduct left behind.

A free standing riser (FSR) is a combination of a TTR and a flexibleriser. The FSR is a buoyancy can supported TTR that is located outsideof the host facility. Another difference is that the top of the FSR'sair can is located well below the mean water level, approximately fivehundred feet. A flexible flowline is then attached from the top of theFSR to the host facility. This riser concept has a couple of benefits.Host facility motions are decoupled from the riser via the flexibleflowline. Another benefit is that the payload imparted on the hostfacility is small because only approximately one thousand feet offlexible flowline is hanging from the host facility. The flexibleflowline just needs to be separated from the free standing riser andhung off once the host facility is in place.

It can be seen that TTR's have some disadvantages. For example, if thewells that the TTR's are producing from have their production depletebefore the design life of the riser is up, the TTR cannot readily bemoved to accommodate another wellhead, as described above. Because TTR'shave not been designed to be tied in to productions from wellheadsbeyond the normal reach of a TTR, the useful life of TTR's can belimited. This results in the need to have other types of risers, asdescribed above, which adds to the complexity and cost of an offshorehost facility. The TTR used to provide direct access to a well must besupported in the interior of the host facility because the interventionrequires heavy equipment that is moved around on the facility's workingplatform. The tensioning device used to support the riser is either abuoyancy can attached to the top of the riser and floating inside anopen well in the host facility or a mechanical tensioner hangingdirectly from the host facility. The tensions in the individual risersmust be controlled because of varying weights of contents in the riseras it is subjected to different modes of operation. As water depthincreases the length of the riser and thus its weight also increases.Even in shallower depths, where wells produce high pressure fluids athigh temperatures the risers require very thick walls and thus becomeheavier than normal TTR's. In these cases, the traditional buoyancy canand tensioners alone are not adequate to support and control the risertension as required. There is need to be able to carry out this functionusing equipment that is currently proven.

SUMMARY OF THE INVENTION

The invention addresses the above need. What is provided is a means forapplying a controlled tension to a top tension riser. The inventioncombines a mechanical tensioner device and buoyancy can to applycontrolled tension to an individual riser or group of risers supportedby a floating offshore structure. The buoyancy can applies statictension force on the riser(s). The mechanical tensioner appliesadditional tension force to assist the can in limiting the stroke of theriser as the supporting structure is displaced from its nominalposition.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention reference should be made to the following description, takenin conjunction with the accompanying drawings in which like parts aregiven like reference numerals, and wherein:

FIG. 1 is a side sectional view of the invention.

FIG. 2 is a top view of the invention.

FIG. 3 is a more detailed side sectional view of the invention.

FIG. 4 is a side sectional view of an alternate embodiment of theinvention.

FIG. 5 is a more detailed side sectional view of the alternateembodiment of FIG. 4 that eliminates some of the surrounding structurefor ease of understanding.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, it is seen in FIGS. 1 and 2 that the means 10for applying tension to a top tension riser is generally comprised of amechanical tensioning device 12 mounted on a floating offshore structure14 such as a spar type structure in combination with a buoyancy can 16to support a riser 18 for a dry tree 20.

The mechanical tensioning device 12 is rigidly mounted on the floatingoffshore structure 14 adjacent the center well 15. The mechanicaltensioning device 12 includes a hydraulic cylinder 22 and hydraulic ram(stroke tensioner) 24 movably received in the cylinder 22. A line 26 isconnected to the ram 24 and buoyancy can 16. The line is received on asheave 28 mounted on the offshore structure 14. The stroke tensioner maybe pressure compensated to restrict the stroke length to a predeterminedstroke length. As seen in FIGS. 1 and 2, more than one mechanicaltensioning device 12 may be mounted around the center well 15 asrequired.

FIG. 3 illustrates an example where the hydraulic ram 24 is shown in itsnormal operating position, indicated by arrow 30, and in a secondtensioning position, indicated by arrow 32, in response to movement bythe offshore structure 14.

The buoyancy can 16 operates in combination with the mechanicaltensioning device 12 via line 26 which is connected between the two.

When the offshore structure 14 is installed at the offshore site and allequipment is operational, the invention operates as follows. When theoffshore structure 14 is in the ideal position above the sea floor, thehydraulic ram 24 is in its normal operating position as indicated byarrow 30. As the offshore structure 14 moves horizontally in response toenvironmental forces, the riser 18 offsets, tends to pull downward, andforms a catenary curve in the riser 18. The top end of the riser 18pulls down because of the weight of the catenary curve and thus pullsthe buoyancy can 16 down until equilibrium is established. If thebuoyancy can 16 provides the only top tension on the riser 18, it ispossible that the riser will pull down below the limit that will resultin damage to the riser 18. The mechanical tensioning device 12, via theline connected to the buoyancy can 16, acts to limit the pull down ofthe riser 18 before it reaches the damage point by providing tension onthe line 26 as the hydraulic ram moves toward the predetermined limit ofits second tensioned operating position indicated by arrow 32.

The stroke of the mechanical tensioning device 12 may be adjusted asnecessary. As an example, FIG. 3 shows a hydraulic ram 24 with a normalfull fifteen foot stroke that has been pressure compensated to a tenfoot stroke to meet the requirements of the particular offshorestructure, riser, and buoyancy can arrangement.

While only a single buoyancy can is shown, it should be understood thatthe invention is applicable where there are multiple buoyancy cans. Thecables from the tensioning devices are attached to the buoyancy can.

FIGS. 4 and 5 illustrate an alternate embodiment of the invention wherethe mechanical tensioning devices 12 are mounted on a tensioner supportstructure 34 instead of directly to the offshore structure. It is seenthat the tensioner support structure 34 is attached to the offshorestructure 14 and spans the center well 15. The operational principle isthe same.

The invention provides several advantages.

It enables the use of risers that are normally too heavy or experiencetoo much stroke for mechanical tensioners or buoyancy cans alone becauseof the riser's function or water depth of deployment.

It eliminates the requirement for redundancy capacity in buoyancy cansand tensioners.

It reduces stroke ranges as compared to buoyancy cans alone.

It reduces the tension as compared to the use of tensioners alone.

Because many varying and differing embodiments may be made within thescope of the inventive concept herein taught and because manymodifications may be made in the embodiment herein detailed inaccordance with the descriptive requirement of the law, it is to beunderstood that the details herein are to be interpreted as illustrativeand not in a limiting sense.

1. In an offshore floating structure having one or more top tensionedrisers, a means for applying tension to the risers, comprising: a. amechanical tensioning device supported by the offshore floatingstructure; b. a buoyancy can supporting the riser; and c. a lineconnecting said mechanical tensioning device to said buoyancy can. 2.The tension applying means of claim 1, wherein said mechanicaltensioning device is attached directly to the offshore floatingstructure.
 3. The tension applying means of claim 1, wherein saidmechanical tensioning device is attached to a tensioner supportstructure mounted on the offshore floating structure.
 4. The tensionapplying means of claim 1, wherein said mechanical tensioning devicecomprises a hydraulic cylinder and ram.
 5. A method for applying tensionto top tension risers on a floating offshore structure, comprising: a.providing a buoyancy can supporting the riser; b. providing a mechanicaltensioning device supported by the offshore floating structure; and c.attaching a line between said buoyancy can and said mechanicaltensioning device such that said buoyancy can and mechanical tensioningdevice work in combination to support the riser.